<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input {
            color: #999;
        }
    </style>
</head>

<body>
    <input type="text" name="" id="" value="手机">
    <script>
        var input = document.querySelector("input");
        input.onfocus = function() {
            if (this.value == "手机") {
                input.va1lue = "";
            }
            this.style.color = "#333"
        }
        input.onblur = function() {
            if (this.value == "") {
                input.value = "手机";
            }
            this.style.color = "#999";
        }
    </script>
</body>

</html>